The present invention provides a method of screening for compounds which inhibit steroid sulfatase activity related to the biosynthesis of steroid hormones with the aim of healing hormone-dependent diseases such as breast cancer.
It has been known for long that anti-hormone agents are effective against hormone-dependent cancers, and anti-male sex hormone agents and anti-female sex hormone agents have been used for treating them. Recently, tamoxifen, which is an anti-hormone agent, has been developed as an anti-tumor agent for hormone-dependent cancers (Iacobelli, S., et al. , The Role of Tamoxifen in Breast Cancer, Raven Press, NY, 1982). For the biosynthesis of steroid hormones, specifically, estrogen, two pathways are known; i.e., the aromatase pathway through which androstenedione is converted into estrone and the sulfatase pathway through which estrone-sulfate is converted into estrone. In recent years, rapid progress has been made in the research and development of aromatase inhibitors which suppress the estrone biosynthesis by blocking the aromatase pathway.
For example, there have been reported screening systems using, as markers, the inhibition of the growth of cells carrying an introduced aromatase gene [Cancer Res., 50, 6949 (1990); J. Steroid Biochem. Molec. Biol., 44, 611 (1993); PCT National Publication No. 502261/92] and the reduction in the volume of a tumor established by transplanting cells carrying an introduced aromatase gene into a nude mouse [Cancer Res., 55, 3073 (1995)].
However, with the development of analytical research on the steroid hormone biosynthetic systems, it has become clear that the steroid sulfatase pathway plays a more important role than the aromatase pathway in the estrone biosynthesis [J. Clin. Endocrinol. Metab., 59, 29 (1984); Ann. NY Acad. Sci., 464, 126 (1986); J. Steroid Biochem., 34, 155 (1989)].
Recently, reports have been made of attempts to screen steroid sulfatase inhibitors by assay systems using a fraction having steroid sulfatase activity obtained by fractionating human placenta rich in steroid sulfatase [Steroids, 58, 106 (1993); J. Steroid Biochem. Molec. Biol., 48, 523 (1994); J. Steroid Biochem. Molec. Biol., 59, 41 (1996); Biochemistry, 36, 2586 (1997)]. It is also reported that steroid sulfatase inhibitors can be screened and evaluated by not only the inhibiting activity on a fraction having the enzyme activity, but also the growth inhibition against cells, showing hormone-dependent growth [J. Steroid Biochem. Molec. Biol., 59, 83 (1996); Cancer Res., 57, 702 (1997)]. A steroid sulfatase gene derived from human is known [J. Biol. Chem., 264, 13865 (1989)].
However, the screening systems using a fraction having the enzyme activity and cells showing hormone-dependent growth are not sufficient in sensitivity. Further, there exists a need for a screening system using animals which can be models of actual conditions of diseases.
An object of the present invention is to provide a method of efficiently screening for compounds inhibiting steroid sulfatase activity which are useful for the treatment of hormone-dependent diseases such as breast cancer.
The present invention relates to a method of screening for steroid sulfatase inhibitors by using the inhibiting activity on the growth of cells carrying an introduced steroid sulfatase gene as a marker. More specifically, the present invention relates to a method of screening for steroid sulfatase inhibitors by causing a test compound to act on cells carrying an introduced steroid sulfatase gene and estimating the inhibiting activity of the compound on the growth of the cells. The present invention also relates to a method of screening for steroid sulfatase inhibitors by transplanting cells carrying an introduced steroid sulfatase gene into an animal, causing a test compound to act on the animal, and estimating the inhibiting activity of the compound on the growth of the cells. Further, the present invention relates to steroid sulfatase inhibitors obtained by the above screening methods. However, known steroid sulfatase inhibitors are excluded from the above-mentioned steroid sulfatase inhibitors which are one of the aspects of the present invention.
In the present invention, any gene can be used that encodes a polypeptide having steroid sulfatase activity. Examples of suitable genes include a gene encoding a polypeptide comprising the amino acid sequence shown in SEQ ID NO: 1, and a gene encoding a polypeptide which has an amino acid sequence wherein one or more amino acid residues are deleted, substituted or added in the amino acid sequence of a polypeptide having the amino acid sequence shown in SEQ ID NO: 1 and which has steroid sulfatase activity.
The polypeptides having an amino acid sequence wherein one or more amino acid residues are deleted, substituted or added in the amino acid sequence of the polypeptide and having steroid sulfatase activity can be prepared according to the methods described in Nucleic Acids Research, 10, 6487 (1982); Proc. Natl. Acad. Sci., USA, 79, 6409 (1982); Proc. Natl. Acad. Sci., USA, 81, 5662 (1984); Science, 224, 1431 (1984); WO85/00817; Nature, 316, 601 (1985); Gene, 34, 315 (1985); Nucleic Acids Research, 13, 4431 (1985); Current Protocols in Molecular Biology, the 8th chapter, Mutagenesis of Cloned DNA, John Wiley and Sons, Inc.; 1989, etc.
Specific examples of the genes encoding a polypeptide having steroid sulfatase activity useful in the present invention include DNA comprising the nucleotide sequence shown in SEQ ID NO: 2, and DNA which hybridizes to such DNA under stringent conditions and which encodes a polypeptide having steroid sulfatase activity.
The xe2x80x9cDNA which is hybridizable under stringent conditionsxe2x80x9d refers to DNA which is obtained by colony hybridization, plaque hybridization or Southern blot hybridization using the DNA encoding a polypeptide having steroid sulfatase activity as a probe. Such DNA can be identified, for example, by performing hybridization at 65xc2x0 C. in the presence of 0.7-1.0 M sodium chloride using a filter with colony- or plaque-derived DNA immobilized thereon and then washing the filter at 65xc2x0 C. using 0.1 to 2-fold concentrated SSC solution (SSC solution: 150 mM sodium chloride and 15 mM sodium citrate) The hybridizable DNA is, for example, DNA having at least 60% homology, preferably 80% or more homology, more preferably 95% or more homology to the nucleotide sequence of DNA encoding a polypeptide having an amino acid sequence included in the amino acid sequence shown in SEQ ID NO: 1.
Hybridization can be carried out according to the methods described in Sambrook, Fritsch and Maniatis, edit., Molecular Cloning, A Laboratory Manual, Second Edition, Cold Spring Harbor Laboratory Press, 1989 (hereinafter abbreviated as Molecular Cloning, Second Edition), etc.
As the host cell for the introduction of the steroid sulfatase gene of the present invention, any bacterial cells, yeast cells, animal cells, insect cells, etc. can be used so far as they are capable of introduction of a steroid sulfatase gene and expression of steroid sulfatase. Preferred are cells showing hormone-dependent growth.
Of steroid sulfatases of the present invention, human-derived steroid sulfatase is reported to require post-translational modifications for the expression of the activity [Cell, 82, 271 (1998)], and in this case, an animal cell is used as the host cell. Examples of animal cells useful as the host cell include Namalwa cells, HBT5637 (Japanese Published Unexamined Patent Application No. 299/88), COS1 cells, COS7 cells and CHO cells. When cells carrying an introduced steroid sulfatase gene are transplanted into an animal such as a mouse and fixed therein to form a tumor, cells which show hormone-dependent growth and which are easily fixed in an animal to form a tumor are used. Suitable cells include MCF-7 [Int. J. Cancer, 54, 119 (1993)], T47D [J. Clin. Endocrinol., 55, 276 (1982)] and Ishikawa Strain [J. Steroid Biochem., 24, 85 (1986)].
When an animal cell is used as the host cell, the expression vectors that can be employed are those capable of autonomous replication or integration into chromosome in the host cell and comprising a promoter at a position appropriate for the transcription of a steroid sulfatase gene. Suitable vectors include pcDNAI, pcDNA3 and pcDM8 (all produced by Invitrogen), pAGE107 [Japanese Published Unexamined Patent Application No. 22979/91; Cytotechnology, 3, 133 (1990)], pAS3-3 (Japanese Published Unexamined Patent Application No. 227075/90), pCDM8 [Nature, 329, 840 (1987)], pcDNAI/Amp (Invitrogen), pREP (Invitrogen), pAGE103 [J. Biochem., 101, 1307 (1987)], etc.
As the promoter, any promoters capable of functioning in animal cells can be used. Suitable promoters include the promoter of IE (immediate early) gene of cytomegalovirus (human CMV), SV40 early promoter, metallothionein promoter, the promoter of a retrovirus, heat shock promoter, SR xcex1 promoter, etc. The enhancer of IE gene of human CMV may be used in combination with the promoter.
Introduction of the recombinant vector into animal cells can be carried out by any of the methods for introducing DNA into animal cells, for example, electroporation [Cytotechnology, 3, 133 (1990)], the calcium phosphate method (Japanese Published Unexamined Patent Application No. 227075/90) and lipofection method [Proc. Natl. Acad. Sci., USA, 84, 7413 (1987)]. Transformed cells can be obtained and cultured according to the method described in Japanese Published Unexamined Patent Application No. 227075/90 or Japanese Published Unexamined Patent Application No. 257891/90.
When an insect cell is used as the host cell, steroid sulfatase can be expressed in the insect cell by using the methods described in Baculovirus Expression Vectors, A Laboratory Manual, W. H. Freeman and Company, New York (1992); Current Protocols in Molecular Biology, Supplement 38, 28, Unit 16.9, 16.11, John Wiley and Sons, New York (1995); Bio/Technology, 6, 47 (1998), etc. That is, a vector carrying an introduced steroid sulfatase gene and a baculovirus are cotransfected into an insect cell, the insect cell is cultured to obtain a recombinant virus from the culture supernatant, and then an insect cell is infected with the recombinant virus, whereby steroid sulfatase can be expressed in the insect cell.
Examples of the vectors suitable for use in this method are pVL1392, pVL1393 and pBlueBacIII (all produced by Invitrogen). An example of the baculovirus is Autographa californica nuclear polyhedrosis virus, which is a virus infecting insects belonging to the family Barathra.
Examples of the insect cells useful as the host cell are Sf9 and Sf21 which are ovarian cells of Spodoptera frugiperda (Baculovirus Expression Vectors, A Laboratory Manual, W. H. Freeman and Company, New York, 1992) and High 5 which is ovarian cells of Trichoplusia ni (Invitrogen).
Cotransfection of the vector carrying the introduced recombinant gene and the baculovirus into an insect cell for the preparation of the recombinant virus can be carried out by the calcium phosphate method (Japanese Published Unexamined Patent Application No. 227075/90), lipofection method [Proc. Natl. Acad. Sci., USA, 84, 7413 (1987)], etc.
When a yeast cell is used as the host cell, YEp13 (ATCC 37115), YEp24 (ATCC 37051), YCp (ATCC 37419), etc. can be used as the expression vector.
As the promoter, any promoters capable of functioning in yeast cells can be used. Suitable promoters include PHO5 promoter, PGK promoter, GAP promoter, ADH promoter, gal1 promoter, gal10 promoter, heat shock protein promoter, MF xcex11 promoter, CUP1 promoter, etc.
Examples of the yeast cells useful as the host cell include cells of Saccharomyces cerevisiae, Schizosaccharomyces pombe, Kluyveromyces lactis, Trichosporon pullulans and Schwanniomyces alluvius. 
Introduction of the recombinant vector can be carried out by any of the methods for introducing DNA into yeast cells, for example, electroporation [Methods. Enzymol., 194, 182 (1990)], the spheroplast method [Proc. Natl. Acad. Sci., USA, 84, 1929 (1978)] and the lithium acetate method [J. Bacteriol., 153, 163 (1983); Proc. Natl. Acad. Sci., USA, 75, 1929 (1978)].
When the gene is expressed in a yeast cell, an animal cell or an insect cell, a glycosylated polypeptide can be obtained.
When a procaryotic cell such as a bacterial cell is used as the host cell, it is preferred that the expression vector for the steroid sulfatase gene should be capable of autonomous replication in the host cell and that the expression vector should comprise a promoter, a ribosome binding sequence, DNA encoding steroid sulfatase and a transcription termination sequence. The vector may further comprise a gene regulating the promoter.
Examples of suitable expression vectors are pBTrp2, pBTac1 and pBTac2 (all produced by Boehringer Mannheim), pSE280 (Invitrogen), pGEMEX-1 (Promega), pQE-8 (QIAGEN), pKYP10 (Japanese Published Unexamined Patent Application No. 110600/83), pKYP200 [Agric. Biol. Chem., 48, 669 (1984)], pLSA1 [Agric. Biol. Chem., 53, 277 (1989)], pGEL1 [Proc. Natl. Acad. Sci., USA, 82, 4306 (1985)], pBluescript (Stratagene), pTrs30 (FERM BP-5407), pTrs32 (FERM BP-5408), pGHA2 (FERM BP-400), pGKA2 (FERM BP-6798), pTerm2 (Japanese Published Unexamined Patent Application No. 22979/91, U.S. Pat. No. 4,686,191, U.S. Pat. No. 4,939,094, U.S. Pat. No. 5,160,735), pKK233-2 (Pharmacia), pGEX (Pharmacia), pET system (Novagen), pUB110 [described in Recombinant DNA Techniques (1983), Addison-Wesley Pub. Co.] and pSupex [J. Bacteriol., 172, 2392 (1990)].
As the promoter, any promoters capable of functioning in host cells such as Escherichia coli can be used. For example, promoters derived from Escherichia coli or phage, such as trp promoter (Ptrp), lac promoter (Plac), PL promoter, PR promoter, PletI promoter and PSE promoter, SPO1 promoter, SPO2 promoter and penP promoter can be used. Artificially designed or modified promoters such as a promoter in which two Ptrp are combined in tandem (Ptrpxc3x972) and tac promoter, etc. can also be used.
It is preferred to use a plasmid in which the distance between the Shine-Dalgarno sequence (ribosome binding sequence) and the initiation codon is adjusted to an appropriate length (e.g., 6-18 bases).
The transcription termination sequence is not essential for the expression of the steroid sulfatase gene of the present invention, but it is preferred that the transcription termination sequence lie immediately downstream of the structural gene.
Examples of the procaryotes useful as the host cell are microorganisms belonging to the genera Escherichia, Serratia, Corynebacterium, Brevibacterium, Pseudomonas, Bacillus and Microbacterium, specifically, Escherichia coli XL1-Blue, Escherichia coli XL2-Blue, Escherichia coli DH1, Escherichia coli MC1000, Escherichia coli KY3276, Escherichia coli W1485, Escherichia coli JM109, Escherichia coli HB101, Escherichia coli No. 49, Escherichia coli W3110, Escherichia coli NY49, Serratia marcescens OUT8259, Pseudomonas putida ATCC 12633, Bacillus subtilis ATCC 33677, Bacillus amyloliquefaciens, Brevibacterium ammoniagenes, Brevibacterium immariophilum ATCC 14068, Brevibacterium saccharolyticum ATCC 14066, Brevibacterium flavum ATCC 14067, Brevibacterium lactofermentum ATCC 13869, Corynebacterium glutamicum ATCC 13032, Corynebacterium acetoacidophilum ATCC 13870 and Microbacterium ammoniaphilum ATCC 15354. Preferred are Escherichia coli XL1-Blue, Escherichia coli XL2-Blue, Escherichia coli DH1 and Escherichia coli MC1000.
Introduction of the recombinant vector can be carried out by any of the methods for introducing DNA into the above host cells, for example, the method using calcium ion [Proc. Natl. Acad. Sci., USA, 69, 2110 (1972)] and the protoplast method [Japanese Published Unexamined Patent Application No. 248394/88; Gene, 17, 107 (1982); Molecular and General Genetics, 168, 111 (1979)].
Introduction of the steroid sulfatase gene into the above host cells can be carried out by the methods described in Molecular Cloning, Second Edition; Current Protocols in Molecular Biology, Supplement 1-34, John Wiley and Sons, New York (1995), etc. That is, DNA encoding steroid sulfatase is digested with a restriction enzyme or deoxyribonuclease, and the obtained DNA fragment containing the DNA encoding steroid sulfatase is inserted downstream of a promoter in an expression vector. Then, the expression vector containing the DNA is introduced into host cells, followed by selection of transformed cells carrying the introduced expression vector containing the steroid sulfatase gene.
Selection of cells carrying the introduced steroid sulfatase gene is carried out using the expression of the activity of a marker gene on the expression vector and the improvement of steroid sulfatase activity as markers. Measurement of the steroid sulfatase activity is carried out according to the method of Reed, et al. [Int. J. Cancer, 50, 901 (1992)].
For the culturing of the transformed cells prepared by using animal cells as the host cells, any media that can be assimilated by the animal cells can be used. Examples of suitable media include those generally used for culturing animal cells such as RPMI1640 medium [The Journal of the American Medical Association, 199, 519 (1967)], Eagle""s MEM [Science, 122, 501 (1952)], DMEM [Virology, 8, 396 (1959)] and 199 medium [Proceeding of the Society for the Biological Medicine, 73, 1 (1950)], and media prepared by adding fetal calf serum or the like to these media.
Culturing is usually carried out at pH 6-8 at 30-40xc2x0 C. for 1-7 days in the presence of 5% CO2. If necessary, antibiotics such as kanamycin and penicillin may be added to the medium during the culturing.
Fetal calf sera which may be added to the medium are those containing no steroid in view of the examination of the hormone-dependent growth of cells. For example, a fetal calf serum freed of steroid compounds by treatment with activated carbon can be used. Commercially available ones such as Steroid-free calf serum (Hyclone, Calif.) can also be used.
As the selective marker for transformed cells, those existing on the respective expression vectors are used. Suitable markers include genes responsible for the resistance to hygromycin, G418, methotrexate, etc. Transformed cells can be selected based upon the appearance of resistance to these drugs caused by the expression of these selective markers. Genes coding for enzymes such as glutamine synthetase can also be used as selective markers.
Then, transformed cells are selected based upon the increase in steroid sulfatase activity. Measurement of the steroid sulfatase activity can be carried out according to the method of Vaccaro, et al. [Enzyme, 37, 115 (1987)] or that of Reed, et al. [Int. J. Cancer, 50, 901 (1992)] based upon the detection of estrone in a toluene-extracted fraction resulting from the conversion of tritium-labeled estrone-3-sulfate. The enzyme activity can also be measured using synthetic substrates such as 4-methyl-umbelliferyl sulfate [Experimentia, 35, 309 (1979)] and p-nitrophenyl sulfate [Padiat. Res., 11, 894 (1977)] in place of estrone-3-sulfate.
Examination of the hormone-dependent cell growth can be carried out by first culturing cells in an estrogen-free medium, then subculturing the cells in media containing estrogen compounds such as estrone-3-sulfate, estradiol and estrone at various concentrations, and observing the cell growth in the presence of each estrogen compound at each concentration to examine the dependence of the cell growth on the concentration of an estrogen compound.
It is preferred to use reagents of high purity as estrogen compounds. Specifically, in the case of estrone-3-sulfate, it is preferably used after being dissolved in distilled water, extracted at least twice with ether, and then freed of ether-soluble impurities.
Culturing of the transformed cells of the present invention can be carried out according to a usual method for culturing the host cells.
For the culturing of the transformed cells prepared by using insect cells as the host cells, any media that can be assimilated by the insect cells can be used. Examples of suitable media include those generally used for culturing insect cells such as TNM-FH medium (Pharmingen), Sf-900 II SFM medium (GIBCO) ExCell400 and ExCell405 (both produced by JRH Biosciences) and Grace""s Insect Medium [Grace, T.C.C., Nature, 195, 788 (1962)]. Culturing is usually carried out at pH 6-7 at 25-30xc2x0 C. for 1-5 days. If necessary, antibiotics such as gentamicin may be added to the medium during the culturing.
For the culturing of the transformed cells prepared by using procaryotic cells such as Escherichia coli cells or eucaryotic cells such as yeast cells as the host cells, any of natural media and synthetic media can be used insofar as it is a medium suitable for efficient culturing of the transformed cells which contains carbon sources, nitrogen sources, inorganic salts, etc. that can be assimilated by the host used.
Examples of suitable carbon sources include carbohydrates such as glucose, fructose, sucrose, molasses containing them, starch and starch hydrolyzate; organic acids such as acetic acid and propionic acid; and alcohols such as ethanol and propanol.
As the nitrogen sources, ammonia, ammonium salts of inorganic or organic acids such as ammonium chloride, ammonium sulfate, ammonium acetate and ammonium phosphate, and other nitrogen-containing compounds can be used as well as peptone, meat extract, yeast extract, corn steep liquor, casein hydrolyzate, soybean cake, soybean cake hydrolyzate and various fermented cells and digested products thereof.
Examples of the inorganic salts include potassium dihydrogenphosphate, dipotassium hydrogenphosphate, magnesium phosphate, magnesium sulfate, sodium chloride, ferrous sulfate, manganese sulfate, copper sulfate and calcium carbonate.
Culturing is usually carried out under aerobic conditions, for example, by shaking culture or submerged spinner culture under aeration, at 15-40xc2x0 C. for 16-96 hours. The pH is maintained at 3.0-9.0 during the culturing. The pH adjustment is carried out by using an organic or inorganic acid, an alkali solution, urea, calcium carbonate, ammonia, etc. If necessary, antibiotics such as ampicillin and tetracycline may be added to the medium during the culturing.
When a microorganism transformed with an expression vector comprising an inducible promoter is cultured, an inducer may be added to the medium, if necessary. For example, in the case of a microorganism transformed with an expression vector comprising lac promoter, isopropyl-xcex2-D-thiogalactopyranoside or the like may be added to the medium; and in the case of a microorganism transformed with an expression vector comprising trp promoter, indoleacrylic acid or the like may be added.
Screening for steroid sulfatase inhibitors can be carried out by causing a test compound to act on the transformed cells obtained above and estimating the inhibiting activity of the compound on the growth of the cells. The transformed cells to be acted on by a test compound may be either those cultured in vitro or those transplanted into an animal.
Screening by the use of the cells cultured in vitro can be carried out in the following manner.
First, the number of living transformed cells in a test tube is estimated and then a test compound is added to the cells. After culturing for a certain period, the number of living cells is estimated. The steroid sulfatase-inhibiting activity of the test compound can be estimated by comparing the cell growth in the presence of the test compound with that in the absence of the test compound.
Screening by the use of the animal cells transplanted into an animal can be carried out in the following manner.
First, cells carrying the introduced steroid sulfatase gene are transplanted into an animal such as a mouse for fixation to form a tumor. That is, tumor cells carrying the introduced steroid sulfatase gene are transplanted subcutaneously into an immunodeficient mouse, for example, a BALB/c-nu/nu (a nude mouse) or an SCID mouse in an amount of 1-5xc3x97106 cells to form a tumor. At the time of transplantation, a crude extract of basement membrane containing adhesion factor (e.g., matrigel-basement membrane, Becton and Dickinson) may be added in order to improve the fixation rate [Br. J. Cancer, 67, 953 (1993)]. In the case of estrogen-dependent breast cancer, the fixation rate is improved and the tumor growth after the fixation is promoted by administering an estrogen compound.
Screening for steroid sulfatase inhibitors by the use of the animal cells can be carried out in the following manner.
The volume of a tumor can be calculated from the shorter diameter and the longer diameter of the tumor measured with slide calipers according to the following approximation formula.
Tumor volume=(longer diameter)xc3x97(shorter diameter)2÷2
Animals in which the tumor growth was confirmed after administration of estrone-3-sulfate are selected, and a test compound is administered thereto. Then, the tumor volume is measured to estimate the steroid sulfatase-inhibiting activity of the test compound. The test compound may be administered by any of intravenous, subcutaneous, intraperitoneal and oral routes. The steroid sulfatase-inhibiting activity of the compound can be estimated by measuring the tumor volume at intervals of 3-4 days after the administration, and then comparing the tumor growth between the group to which only estrone-3-sulfate was administered and the group to which estrone-3-sulfate and the test compound were administered.
Certain embodiments of the present invention are illustrated in the following examples. When a kit was used in the following examples, the experiment was carried out according to the attached protocol unless otherwise specified.